In relation to protein turnover or aging, we have studied enzyme inactivation associated with oxidative modification of enzyme protein. Many enzymes including E. coli glutamine synthetase undergo oxidative inactivation in vitro by a variety of mixed-function oxidation systems generating activated oxygen species. A prominent change on oxidative inactivation is formation of carbonyl protein derivative which can react with various carbonyl reagents. The erythrocyte is a very good model for study of protein degradation because it is no longer active in protein biosynthesis. Erythrocytes are also a good model for aging because they can be easily separated by age using density gradient sedimentation. We have fractionated erythrocytes according to age and measured the carbonyl content of purified enzymes.